Page:Popular Science Monthly Volume 21.djvu/597

Rh sufficient to maintain from sixteen thousand to eighteen thousand sixteen-candle lamps. Taking the former figure, this is one and a quarter mile per 1,000 lamps. Basing the calculation for mains upon this mileage and the size of the present mains, the same number of miles of electric mains would be required as for gas. The present conductors are, as stated, in the form of half-round copper rods, of varying sizes, diminishing of course as they proceed from the station. They are, however, equivalent to round rods with a uniform diameter of one half inch. Such rods weigh of a pound per foot, and 3986·4 pounds per mile, costing, at 28 cents per pound, 81,116 per mile. As there are two rods in each main, the cost per mile for copper would be $2,232. To this must be added $1,200 per mile for wrought-iron tube, boxes at the joints between the mains and house wires, and insulating material, and $1,000 per mile for laying, making the total cost of the main per mile, laid ready for use, $4,432. Four fifths of the mains would be of this size, the other fifth being feeders equivalent to round rods three fourths of an inch in diameter. These latter weigh 1·69 pound per foot, and would therefore cost $2,340 per mile, and, taking the cost of inclosing tube, insulation, and laying the same as above, their total cost per mile would be $7,196. The total cost of the mains, forty miles at $4,432 per mile, and ten miles at $7,196 per mile, would therefore amount to the same as the gas mains, viz., $250,000. If real estate be added at $50,000, which in most cities requiring this size of plant would be ample, the total cost of the electric plant would be the same as one for gas.

The elements entering into the cost of the light to the company furnishing it are, in each case, the interest on the investment, depreciation, or the amount spent each year in keeping the property in good condition, the labor of all kinds—in the manufacture, distribution, and management—and lastly the cost of the materials used in its production. In the case of gas but a few of these items as they occur in American works are obtainable, so that recourse must be had to the published reports of foreign companies, and the like items estimated for this country. Of these, the reports of the London companies as analyzed by Mr. Field will best serve for the purpose of the present comparison. Taking first the item of depreciation, we find that for the four metropolitan companies this was, for the year 1880, on the producing portion of the plant 9·86 cents per 1,000 feet of gas sold, or about five and a half per cent on the cost of this part of the plant as it has been taken in this paper. Calling this ten cents a thousand